ROME, Italy (Reuters) -- Some 13 million hectares of forests are destroyed around the world each year, an area the size of Greece, although the net loss of trees has finally slowed thanks mainly to new plantations, the United Nations said on Monday.The U.N. Food and Agriculture Organization (FAO) said its Global Forest Resources Assessment was the most exhaustive such survey undertaken, covering 229 countries and territories.Taking into account plantations, landscape restoration and the natural expansion of some forests, the FAO said the net loss of forest area between 2000-2005 was some 7.3 million hectares a year against 8.9 million hectares in the 1990-2000 period.FAO officials hailed the improvement in the net loss figure, saying China in particular had embarked on a major tree-growing program to provide timber for its construction boom and to tackle the process of deforestation."There are reasons to be very optimistic about what is happening," Hosny El-Lakany, FAO's assistant director general for forestry, told a news conference.However, environmental groups accused the FAO of playing down the devastation of the world's most important forests."FAO continues to emphasize the net forest loss number. This is misleading because most of the world's most valuable forests, especially in the tropics, are vanishing as fast as ever," said Simon Counsell, head of the Rainforest Foundation in Britain."These figures are the main basis for global decision-making on world's most important eco-systems. We fear that bad decisions are going to made on the basis of bad data."FAO said forests covered nearly 4 billion hectares, some 30 percent of the world's land, with 10 countries accounting for two-thirds of all forest area -- Australia, Brazil, Canada, China, the Democratic Republic of Congo, India, Indonesia, Peru, Russia and the United States.South America suffered the largest net loss of forests between 2000 and 2005 -- around 4.3 million hectares per year -- followed by Africa, which lost 4 million hectares annually.By contrast, forest area grew in Europe, although at a slower rate than in the 1990s, while Asia moved from a net loss of some 800,000 hectares a year to a net gain of 1 million a year -- thanks mainly to large scale planting in China.The FAO defines a forest as an area larger than 0.5 hectares where 10 percent of the ground is covered by tree canopy.The Rainforest Foundation said this definition was far too loose. "Ten percent is just land with a few trees dotted around. They are exaggerating the area of forest," Counsell said.The canopy of a tropical forest often covers almost 100 percent of the ground. Environmentalists say when this figure falls below 50 percent, the forest's eco-system is wrecked.But the FAO defended its methodology, saying it was almost impossible to gauge the degradation inside forests, and warned against excessive alarmism.It said primary forests, which are areas undisturbed by humans, represented 36 percent of total global forests, with some 6 million hectares lost or modified each year."It is obviously very sad to lose this amount, but you should bear in mind that it represents just 0.4 percent of total primary forest," said survey co-ordinator, Mette Loyche Wilkie.FAO said plantations accounted for less than 5 percent of all the world's forest areas, while 11 percent of forests were official conservation areas -- up 96 million hectares on 1990.Copyright 2005 Reuters. All rights reserved.This material may not be published, broadcast, rewritten, or redistributed.